K.J. Hendricks

Comparison of velvet- and cesium iodide-coated carbon fiber cathodes

Presents results of an experimental comparison of a velvet cathode- and a carbon fiber cathode-coated with cesium iodide (CsI) salt. Each cathode had a planar geometry, with similar emission areas. The cathodes were tested at electric field strengths of 50 kV/cm at anode-cathode (A-K) gaps of 4.0 cm. The applied voltage had a 1-/spl mu/s duration and the pulser was operated at up to a 1-Hz repetition rate. The system had a low base pressure (<1.0/spl times/10/sup -7/ torr). This paper reports the results and comparisons of experiments on each cathode. We address the current and voltage characteristics, the shot-to-shot reproducibility, the pressure evolution of the diode under 1-Hz operation, and the lifetime of the cathodes.

Significant pulse-lengthening in a multigigawatt magnetically insulated transmission line oscillator

The Air Force Research Laboratory/Phillips Laboratory magnetically insulated transmission line oscillator (MILO) is a gigawatt-class, L-band, high-power microwave tube driven by a 500-kV, 60-kA electron beam. A previous version of this tube generated 1.5 GW pulses, but with significant RF pulse shortening, The paper reports on improvements to the tube that have allowed us to increase the output power by 25% and to increase the RF pulse duration by a factor of two and a half.

Comprehensive diagnostic suite for a magnetically insulated transmission line oscillator

The magnetically insulated transmission line oscillator (MILO) is a gigawatt-class cross-field microwave tube that requires no external magnetic field due to inherent self-magnetic insulation. The tube operates with a 500 kV, 60 kA electron beam which, along with high rf fields, poses quite a challenge for diagnosing the device. We report on the comprehensive set of experimental diagnostics (both beam and microwave) employed in the MILO experiment, and show how these diagnostics, teamed with particle-in-cell computer simulations, have been instrumental in discovering problems with the tube.

Investigation of RF breakdowns on the MILO

Describes a series of experiments performed to isolate the RF breakdown mechanisms in the hard tube magnetically insulated transmission line oscillator (MILO) Experiment at the Air Force Phillips Laboratory, Albuquerque, NM. Specifically, several causes of RF breakdown in the region of the vacuum-air interface and the antenna region have been investigated. These causes are X-ray induced electron emission, VUV and visible photoemission of electrons, and breakdown due to large field stresses in the antenna. Each of these mechanisms has the effect of liberating electrons from a surface in a high field region which then are a seed for a breakdown. This paper discusses measurements in the X-ray, VUV, and visible regimes with support from computer simulation. Also, imagery results are shown, which in conjunction with the computer work, point to the presence of high electric field stresses in the antenna, which cause a subsequent breakdown. In particular, X-rays, VUV, visible light, and plasmas do not seem to be the major source of RF breakdown in this tube.

Increasing the RF energy per pulse of an RKO

The Air Force Research Laboratory RKO source has recently demonstrated the ability to convert electron beam power to RF power until the termination of the electron beam pulse, achieving a power of 1.5 GW at an energy of 170 J. These results represent an increase in power of 25-30% in power and energy extracted from this source. This paper discusses the principal research areas encountered in lengthening the RF pulse (FWHM) from 50 ns to the present 120 ns and the associated increase in the RF energy.

Design and implementation of a new UHV threshold cathode test facility

In support of cathode development at the Air Force Research Laboratory, a new ultra-high vacuum cathode test facility is being assembled to complement the existing repetition-rate test pulser. The existing test bed is a 500 kV, 100 Ohm, 1 microsecond(s) duration pulser capable of firing at up to 1 Hz. The new facility is designed to operate at lower voltages (20 - 200 kv), lower impedance (50 - 75 Ohm), and variable pulse lengths (200 - 800 ns) in a single-shot mode. This Threshold Cathode Test Facility (TCTF) will be used to generate data regarding emission turn-on field strengths, outgassing volumes and constituents, vacuum level effects, and anode effects for a variety of field-emitting and explosive- emitting cathode materials. Presented herein are the design parameters of TCTF including diagnostic capabilities and electrostatic simulations of the diode region both with and without beam current.

Particle-in-Cell (PIC) simulation of CW industrial heating magnetron.

Abstract Modern CW industrial heating magnetrons are capable for producing as high as 300 kW of continuous-wave microwave power at frequencies around 900 MHz and are sold commercially [Wynn et al., 2004]. However, to utilize these magnetrons in some specific research and scientific applications being of interest for the Air Force, the necessary adaptation and redesign are required. It means that the detailed knowledge of principles of their operation and full understanding of how the changes of the design parameters affect their operational characteristics are necessary. We have developed and tested computer model of a 10-vane highpower strapped magnetron, which geometrical dimensions and design parameters are close to those of the California Tube Laboratory’s commercially produced CWM-75/100L tube. The computer model is built by using the 3-D Improved Concurrent Electromagnetic Particle-in-Cell (ICEPIC) code. Simulations of the strapped magnetron operation are performed and the following operational characteristics are obtained during the simulation: frequency and mode of magnetron oscillations, output microwave power and efficiency of magnetron operation, anode current and anode-cathode voltage dynamics. The developed computer model of a non-relativistic high-power strapped magnetron may be used by the industrial magnetron community for designing following generations of the CW industrial heating high-power magnetrons.

ICEPIC Simulation of a Strapped Nonrelativistic High-Power CW UHF Magnetron With a Solid Cathode Operating in the Space-Charge Limited Regime

This paper presents the results of particle-in- cell (PIC) simulations of a strapped nonrelativistic ultrahigh- frequency (890-915 MHz) magnetron whose geometrical and operational parameters are close to the parameters of the high- power industrial heating magnetron producing 75-100 kW of continuous-wave microwave power. Simulations of the magnetron operation are performed without artificial RF priming, but rather in natural conditions, when magnetron oscillations start to grow from electromagnetic “noise.” This approach reveals many important details of the “preoscillating” phase of the magnetron operation. It is found, for example, that the start-up time of the magnetron with a solid cathode, operating in the explosive electron emission mode, is determined by the time needed for the electron cloud formed near the cathode to reach the anode, where the fringing dc electric fields of the periodic anode structure begin to perturb the electron cloud and to facilitate the magnetron oscillations to start to grow. The PIC simulations are performed at one magnetic held (0.238 T) and a range of applied voltages, allowing the magnetron to operate in the π mode characterized by five magnetron spokes and T E51-like mode of the induced electromagnetic held distribution within the resonant system of the ten-cavity magnetron.

ICEPIC Simulation of a Strapped Nonrelativistic High-Power UHF Magnetron With a Transparent Cathode Operating in the Explosive Electron Emission Mode

The operation of a high-power nonrelativistic strapped ultra-high-frequency (UHF) multicavity magnetron with transparent cathode is simulated using the Improved Concurrent Electromagnetic Particle-in-Cell (ICEPIC) code. Results of the simulations are compared with simulations of the same magnetron with a solid cylindrical cathode. Both cathodes are explosive-emission electron sources that operate in the space-charge-limited mode of the electron extraction from the cathode. Simulations are part of an effort searching for methods and technologies that enable an increase of output microwave power of strapped nonrelativistic high-power UHF magnetrons by replacing thermionic direct-heated cathodes with a non-thermionic-emission cathode. Results of the simulations show that the anode current <i>I</i>_a and the output power <i>P</i>_out of the magnetron with the transparent cathode are higher than <i>I</i>_a and <i>P</i>_out of the same magnetron with a solid cathode. At the same time, the electronic efficiency η_e of the magnetron with the transparent cathode is less, and the startup time <i>t</i>_s is shorter than η_e and <i>t</i>_s of the magnetron with the solid cathode.

First multi-cavity magnetrons were built in NII-9, Leningrad, during the spring of 1937 PIC simulations of the first 4-cavity S-band CW magnetron

First solid-anode hole-and-slot magnetrons operating in the centimeter band were built in Leningrad, USSR, by N.F. Alekseev and D.E. Malyarov under the supervision of M.A. Bonch-Bruevich who was the scientific director of the NII-9 and who actually developed the original concept of the multi-cavity magnetrons and made first drafts of those devices. The first 4-cavity magnetron operating at ∼9 cm was able to produce about 300 W of CW microwave power with efficiency of ∼20%. Opera-tional parameters of this magnetron are simulated with modern computer codes using geometrical dimensions published in the first and the only paper “Generation of high-power oscillations with a magnetron in the centimeter band”, which describes the work that was “done during the years 1936 and 1937 for the purpose of producing high-power oscillations in the centimeter-wave band by the use of magnetrons”. The dispersion characteristic showing “cold” resonance frequencies of this magnetron is analyzed using the SUPERFISH code. The operational domain showing both the Hartree and the Hull thresholds voltages of this magnetron is calculated using the “cold” frequencies of the magnetron operation. The PIC simulations of the virtual proto-type of the magnetron are performed using the MAGIC code and optimal anode voltages and magnetic fields determined by the operational domain of the magnetron. Results of the simulations show some inherent features of the first four-cavity solid-anode S-band CW magnetron operation.